The present invention relates to a spark-ignition, air-compressing, internal combustion engine that has direct injection of a major portion of the fuel by means of a jet onto the wall of the combustion chamber provided, in the shape of a body of revolution, in the piston, whereby such a rotary motion is imparted by means known per se to the inflowing air in the direction of the injected fuel jet so as to cause the fuel to be removed gradually in the vapor state from the wall of the combustion chamber and to be mixed with the air. The injection nozzle is located in the cylinder head near the combustion chamber rim, and the spark plug, which is arranged opposite (in plan) the injection nozzle, extends into the combustion chamber at the top dead center position of the piston. The side wall of the combustion chamber--viewed in cross section--is formed by two arcs that blend into each other and have radii of curvature R.sub.1, R.sub.2, the first arc with the radius R.sub.1 extending from a restricted combustion chamber opening down to the maximum combustion chamber diameter D.sub.B, and the second arc with the radius R.sub.2 extending down to the bottom of the combustion chamber and/or blending into the latter, where the maximum combustion chamber diameter D.sub.B amounts to 0.5 to 0.7 times the piston diameter D.sub.K and is located at a defined depth t.sub.D from the piston crown relative to the combustion chamber depth T.sub.B, the ratio of the combustion chamber opening diameter d.sub.H to the maximum combustion chamber diameter D.sub.B lying between 0.85 to 0.95, and the throat depth t.sub.H of the combustion chamber opening being between 0.1 and 0.15 T.sub.B.
An internal combustion engine of this general type was disclosed by DE-PS No. 32 45 780.
One of the main problems in an internal combustion engine of the stratified-charge, spark-ignition type is in ensuring ignition of the air/fuel mixture under all possible operating conditions, be it cold starting, idling, or rated output. This means that the mixture composition present in the space of only a few cubic millimeters between the spark plug electrodes, which are covered by the ignition spark, has to remain sufficiently long within the ignition limits during the sparkover in order to produce a sufficiently large flame for the continued burning of the cylinder charge. Since the air velocity in the combustion chamber, and the injected fuel quantity, vary within wide limits, this problem can be solved only by careful matching-up of quite a number of parameters, such as the shape and relative positions of the combustion chamber, fuel jet, and spark plug electrodes, as well as the intensity of the air swirl and the phase positions of the injection and ignition cycles. In matching up each of these parameters, consideration has to be given to the specific fuel consumption, exhaust gas emissions, and component stress levels.
In the prior art (DE-PS No. 32 45 780), this philosophy applied to a naturally aspirated engine. In applying the concept from the naturally aspirated engine to a more powerful supercharged engine, the degree of difficulty of the previously described optimizing process increases because, in this case, both the injected fuel amount and the air charge involved in the process undergo more pronounced changes than in a naturally aspirated unit (between lower idle and rated output there are larger differentials of injection fuel amounts and air velocities). It was found that the shape of the combustion chamber in the piston crown is of great importance in applying this concept.
DE-PS No. 32 45 780 described the adoption of a shallower shape instead of a spheroidal shape for the combustion chamber. The reasons for this were:
(a) To reduce air squish or compression with a view to improving ignition stability and thermal relief of the combustion chamber throat. PA1 (b) Use of as short an electrode as possible in view of the otherwise existing risk of breakage and deformation, and to reduce burnup which is a function of thermal conditions.
Applied to the supercharged internal combustion engine, the combustion chamber shape and fuel jet orientation of the prior art results in too lean a mixture quality at high speed and low engine loads. One possibility of overcoming this problem would be to locate the point where the fuel jet impinges on the wall closer to the spark plug. This would take care of difficulties at part load, but difficulties in the higher load range which really are substantial would be further increased in view of the greater amount of fuel supplied (rich mixture). It appears preferable, therefore, to overcome the difficulties at part load by increasing the curvature of the combustion chamber wall, with the orientation of the fuel jet unchanged and, in this manner, obtaining a richer mixture near the spark plug. The very pronounced curvature of the combustion chamber wall required for this would result in a large and shallow combustion chamber which is unfavorable for the mixture formation and combustion processes (bearing in mind that the combustion chamber volume should remain the same). The large combustion chamber diameter that results would considerably reduce the intensity of the air swirl which is necessary. Moreover, an excessive fuel film thickness would result at higher engine loads which, as a matter of experience, tends to produce drawbacks through sluggish combustion (too rich a mixture in the higher load range).
It is an object of the present invention, in an internal combustion engine of the initially described general type, to satisfy the enhanced mixture formation requirements in applying the concept to a supercharged engine in all operating ranges, i.e. to avoid too lean or too rich an air/fuel mixture being formed, whereby the abovementioned advantages (a) and (b) are to be further improved and reliable ignition and optimum combustion of the mixture formed are ensured under all conditions.